Fiberoptic clearance detection method and optical fiber probe used therein

ABSTRACT

A method includes transmitting light through at least one transmission optical fiber towards a target, receiving light reflected from the target through at least one signal optical fiber, filtering the received light at least two different wavelengths, and using the filtered light to detect a clearance variation. An optical fiber probe includes a plurality of optical fibers, a moisture-resistant enclosure enclosing the optical fibers, a hydrophobic layer situated over an end of the optical fiber probe for preventing moisture from reaching the optical fibers, and a broadband transmission layer between ends of the optical fibers and the hydrophobic layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/241,527, entitled “FIBEROPTIC CLEARANCE DETECTION SYSTEM AND METHOD,”filed Sep. 30, 2008, which is herein incorporated by reference.

BACKGROUND

The subject matter disclosed herein relates generally to clearancedetection.

Clearances between rotor blade tips and casings in turbomachinery suchas steam turbine engines change during and following transient eventsand affect turbomachinery/engine performance. In a more specificexample, axial and radial clearances inside steam turbines betweenrotors and stators have been limiting factors on size, start-up,loading, and shutdown of steam turbines. Currently, conservativeclearances and transition times are used to minimize contact betweensteam turbine stators and rotors.

It would be desirable for a clearance sensing system to have highmechanical strength to tolerate various vibration, temperature, andpressure conditions in turbomachinery installation and maintenance andto have thermal properties that are insensitive to the environment. Inaddition, turbine clearances are sometimes asymmetric, so it isdesirable that a clearance sensing system measure clearances at multiplelocations. It would also be desirable to have a durable system andtechnique to provide a more compact steam turbine with faster start-upcapabilities.

BRIEF DESCRIPTION

Briefly, in one embodiment disclosed herein, a system for measuringclearance comprises an optical fiber probe comprising a plurality ofoptical fibers, at least one of the optical fibers comprising atransmission fiber and at least one of the optical fibers comprising asignal fiber; a light source for providing light through thetransmission fiber towards a target; filters for discriminating lightfrom the signal fibers, at least two of the filters for filteringdifferent wavelengths; and at least one photodetector for receivingfiltered light from the filters.

In another embodiment disclosed herein, a method comprises transmittinglight through at least one transmission optical fiber towards a target;receiving light reflected from the target through at least one signaloptical fiber; filtering the received light at least two differentwavelengths; and using the filtered light to detect a clearancevariation.

In another embodiment disclosed herein, an optical fiber probe comprisesa plurality of optical fibers, a moisture-resistant enclosure enclosingdistal ends of the optical fibers, and a hydrophobic layer situated overan end of the probe for preventing moisture from reaching the opticalfibers.

In yet another embodiment disclosed herein, a system comprises a steamturbine comprising a rotor and a stator, a moisture resistant opticalfiber probe comprising a plurality of optical fibers, at least one ofthe optical fibers comprising a transmission fiber and at least three ofthe optical fibers comprising signal fibers, a light source forproviding light through the transmission fiber towards the steam turbinerotor; filters for discriminating light from the signal fibers and forfiltering out unwanted light; at least one photodetector for receivingfiltered light from the filters; and a processor for receiving signalsfrom the at least one photodetector and detecting a variation indistance between the steam turbine rotor and the steam turbine stator.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a schematic view of a single-point fiberoptic clearancedetection system in accordance with one aspect of the invention;

FIG. 2 is a schematic view of a multi-point fiberoptic clearancedetection system in accordance with another aspect of the invention;

FIG. 3 is a schematic representation of optical switch time gatingmethod to measure each clearance from an optical fiber probe withmultiple laser wavelength modulation in accordance with one aspect ofthe invention;

FIG. 4 is a sectional view which illustrates an optical fiber probeaccording to one embodiment of the invention;

FIG. 5 is a graph illustrating temperatures in the optical fiber probevs distance from the target;

FIG. 6 is several sectional views illustrating various arrangements oftransmission fibers and signal fibers in accordance with embodiments ofthe invention;

FIG. 7 is a graph illustrating reflected intensities of lights vstransmitted distances through several of the arrangements of FIG. 4;

FIG. 8 is a graph illustrating two clearance sensing points from thehigh-sensitive reflectance signal ranges; and

FIG. 9 is a side view showing a schematic steam turbine componentincorporating the optical fiber probes.

DETAILED DESCRIPTION

As discussed in detail below, embodiments of the invention include asystem and a method for detecting clearance. In one more specificexample, the clearance detection is within a steam turbine. Embodimentsdescribed herein are also applicable and beneficial for other types ofturbomachinery, for example, such as gas turbines, compressors, andgenerators.

FIG. 1 illustrates a schematic view of a clearance detection system 10in accordance with one embodiment of the present invention wherein anoptical fiber probe 20 comprises a plurality of optical fibers 21 and22, at least one of the optical fibers 21 comprising a transmissionfiber and at least one of the optical fibers 22 comprising a signalfiber; a light source 30 for providing light through the transmissionfiber 21 towards a target 80; filters 40 for receiving light from thesignal fiber 22, at least two of the filters 40 for filtering differentwavelengths; and at least one photodetector 50 for receiving filteredlight from the filters 40. As used herein, singular forms such as “a,”“an,” and “the” include plural referents unless the context clearlydictates otherwise.

In one embodiment, the light source 30 comprises a multi-wavelengthlight source such as an RGB emitting laser which is optionally tunable.In this embodiment, the emitted lights are of wavelengths of 405 nm to471 nm, 515 nm to 555 nm, and 635 nm to 780 nm. In this embodiment, thefilters 40 may include a narrow band interference filter 42 fortransmitting light in the range of 405 nm to 471 nm, a red filter 44 fortransmitting light in the range of 635 nm to 780 nm, and a green filter46 for transmitting light in the range of 515 nm to 555 nm. The abovedescribed wavelength ranges of the light source and filters are forpurposes of example only, however it is recommended that the ranges ofthe light source and filters be coordinated.

One or more photodetectors 50 may be present. In one embodiment, aplurality of photodetectors 50 are located behind corresponding filters40 for receiving the filtered light. In another embodiment (not shown)one photodetector is used with a rotating wheel and different filters.In one embodiment, the photodetectors comprise silicon avalanchephotodiodes. In another embodiment, wherein a selected laser wavelengthrange of a light source or filter is longer than 1.0 micrometer, anInGaAs-based photodetector or photodetector array is recommended.

A processor may be used for receiving signals from the at least onephotodetector and detecting a variation in distance between the steamturbine rotor and the steam turbine stator. In one embodiment, theprocessor comprises a signal processor 60 for receiving and processingthe signals, a data processor 70 for processing data from signalprocessor 60. Signals from the processor may then be used by acontroller 90 for steam turbine operation condition control.

The optical fiber probe 20 comprises a reflection based fiber bundle,which comprises at least two fibers, at least one fiber 21 (thetransmission fiber) being designated for light delivering and at leastone other fiber (the signal fiber) 22 being designated for reflectancereceiving as indicated in the FIG. 1. The received signal light is sentto the filters 40 and the photodetectors 50. In one embodiment, thelight is sent through an optical splitter 23. In another embodiment,signal fiber 22 comprises a plurality of signal fibers (such as shown inFIG. 6) that transmit light directly to the filters without the need ofa splitter. The transmission fiber 21 transmits light from the lightsource 30 towards a target 80. The signal fiber 22 transmits signalsreflected from the target 80 to the filters 40. Lights from thephotodetectors 50 may be sent to the signal processor 60 by way of anoptical combiner 24.

For effective transmission of visible to near infrared light, in oneembodiment the fibers 21, 22 inside the optical fiber probe 20 compriseUV-grade quartz fiber having a doped fiber clad, a pure silica fibercore, a fiber core refractive index n_(core), and a fiber cladrefractive index n_(clad), where n_(core)>n_(clad). Doping ions for thefiber clad may comprise, for example, fluorine, chlorine, boron, or anycombination thereof. In one embodiment, the fiber comprises a puresilica core with a fluorine-doped clad. In another embodiment, the fibercomprises a sapphire core with a metalized or polymerized clad. Thefiber core diameter is typically in the range of 50 microns to 62.5microns for quartz fibers and 70 microns to 250 microns for sapphirefibers. Optical fiber connectors 100 may be used to connect the lightsource 30, the probe 20, the optical splitter 23, and the filters 40,respectively.

Multiple probes may optionally share a common light source 30 throughuse of a splitter 110 as shown in FIG. 2 for example. Additionally, eachof the filters 40 can also receive light from one or more than onesignal fiber. FIG. 2 illustrates a system 11 for multi-point clearancedetection in accordance with an embodiment of the present invention, inwhich the optical fiber probes 20 are distributed to different locationsinside a steam turbine through an optical splitter 110. In oneembodiment, the reflectance signals from all probes 20 are recombinedwith an optical combiner 120. Light from the optical combiner 120 issent to the filters 40 and the photodetectors 50 by way of an opticalsplitter 140. Such multiplexing and distribution embodiments may includean optical switch 130 to gate times for sequentially monitoring staticand dynamic clearance at different steam turbine locations 80. Lightsfrom photodetectors 50 may be sent to the signal processor 60 by way ofan optical combiner 150 and are then sent to the data processor 70.

Referring to FIG. 3, the optical switch 130 in FIG. 2 controls detectionof clearance from each optical fiber probe 20 by a time sequence 320 foreach of m optical fiber probes 20. During each pulsed laser lightexposure time T, a time period of T/2m is used for clearance detectionfrom each optical fiber probe 20. During each time gating period T, onlyone optical fiber probe is measuring clearance.

FIG. 4 is a diagrammatic representation of an optical fiber probe 20 asimplemented in the systems 10 and 11 of FIG. 1 and FIG. 2. The opticalfiber probe 20 has an enclosure 25 (or “ferrule”) enclosing thetransmission and the signal fibers, a hydrophobic layer 26 situated overan end of the probe for preventing moisture from reaching and corrodingthe optical fibers, and a seal 27 sealing the fibers, the enclosure andthe hydrophobic layer. The ferrule 25 may comprise an iron alloy such asstainless steel, an austenitic nickel-based superalloy such asInconel™625 alloy (melting point: 2450° F.), or a nickel steel alloysuch as Invar 36 alloy (melting point; 2600° F.). The hydrophobic layer26 may comprise a sapphire plate or alumina (Al₂O₃) coating layer andact as a window for transmitting light from and to the optical fibers. Abroadband transmission layer 28 may be disposed at the distal end of thefibers on either side of the hydrophobic layer and may comprise MgO, forexample. The seal 27 may be made of Pt or Au. In one embodiment, betweenthe fibers and the enclosure 25, there is a gap filled with ceramicbonding material 29. In a more specific embodiment, the gap is 25micrometers, and the bonding material 29 is high-temperature ceramicadhesive comprising alumina/silicate material.

The selection of the adhesive material 29 may be based on the mitigationof the thermal stress between the fibers and the metal enclosure at hightemperatures. In one example, when Inconel™ 625 (coefficient of thermalexpansion (CTE): about 6.4 in/in/° F.) is used as the enclosurematerial, the adhesive material comprises an alumina based adhesivehaving CTEs that are in the range of 4.4 in/in/° F. In another examplewhere Titanium (CTE: about 3.9 in/in/° F.) is used as the enclosurematerial, the adhesive material comprises either boron carbide basedadhesive with a CTE of 2.6 in/in/° F.) or silicon carbide based adhesivewith a CTE of 2.9 in/in/° F. It is useful for the selected adhesivematerials to survive temperatures in excess of 1000° F. temperature.Such adhesives are commercially available with two example suppliersbeing Aramco Products, Inc. and Cotronics Corporation. Even when lowertemperature limits are involved, adhesive materials may still be usedfor providing a stress buffer.

For steam turbine applications, the packaging is designed to enable theoptical fiber probe 20 to operate in moist, corrosive, and hightemperature environments and to resist damage during turbomachineryinstallation and maintenance. A packaging length L (FIG. 4) may beselected to be of sufficient length so that package temperature that isexperienced in the vicinity of the target is sufficiently reduced acrossthe length of package 25 so as to not damage equipment at the other(remote) end of package 25. A sufficient temperature gradient may bedesigned based on package length and materials. In one example, if therotor temperature inside the steam turbine, i.e., the target 80, isabout 1500° F., the packaging length L of the probe 20 is longer thanthe thickness of the stator core. In another example, the optical fiberprobe is long enough so that the temperature around the end of thepackage 25 is decreased to be below 200° F. FIG. 5 is a simulated graphwherein the temperature at the target is about 1500° F., the packagelength L is along the X axis, and the resulting temperature at theremote end of the package is along the Y axis.

The optical probe 20 may comprise many fibers, one or more of which aretransmission fibers used for light delivering and one or more of whichare signal fibers for reflectance receiving. FIG. 6 shows arrangements201-206 of transmission fibers 21 and signal fibers 22 in accordancewith embodiments of the invention. Referring to FIG. 6, the transmissionfibers 21 and the signal fibers 22 according to the arrangements 201-206may be arranged in multiple pairs 201, in a random manner 202, in ahalf/half arrangement 203, in one pair 204, or coaxially 205 or 206, forexample. In a specific embodiment, every two of the six signal fibers 22of the arrangement 206 goes to one of the filters 40 of FIG. 1 and nosplitter is needed. FIG. 7 is a graph showing measured clearanceresponses from different fiber bundle arrangements 202-206 of FIG. 6.Each type of fiber configuration has a unique standoff distance andassociated sensitivity.

Static and dynamic clearance measurements are improved when the opticalfiber probe is set at an optimal distance from the target surface. Sucha distance is defined as a “working point”. The distance or workingpoint may vary depending upon the fiber configuration inside the opticalfiber probe 20. FIG. 8 shows two “working points” for an exemplar probe,defined as front and rear clearance sensing points 207 (1.2 mm) and 208(4.1 mm). The clearance can be measured by setting the probe distanceeither at front clearance sensing point 207 which is of highersensitivity with a shorter standoff (distance) from the target, or atrear clearance sensing point 208 which is of a relative lowersensitivity but with a longer standoff (distance) from the target. Whenthe clearance sensing point is chosen, the static and dynamic clearancescan be measured by the signal intensity or power level. When usingdifferent wavelengths, the signal processing will be used to obtain adifferential of each wavelength signal and the relative change ofdifferent wavelengths.

The standoff distance of an optical fiber probe may be selected at leastin part based upon the fiber arrangement or configuration. For example,as can be seen from the graph of FIG. 7, a fiber bundle with shortstandoff may have a higher sensitivity because of its steep frontresponse curve feature. From clearance probe response curves in FIG. 7and FIG. 8, there are two points that could provide propersensitivities. One is defined as front working point, which is firstposition for setting the optical fiber probe for clearance measurement.The other is defined as the rear working point.

If desired, for clearance sensing at a specific location, two probes canbe used with one at the front working point and the other at the rearworking point with the differentiation between two sensing signals beingused for precise static clearance determination. However, one probeeither at front or at rear working point could be used for dynamicclearance sensing. The selection of a fiber bundle type and workingpoint depends upon the maximum clearance that needs to be measured at aspecific steam turbine location.

In one detection example, intensities of lights in signal fibers areI_(R), I_(G), I_(B), an initial distance between the probe 20 and thetarget 80 is d₀, and the clearance variation Δd between earlier time t₁and later time t₂ is determined respectively by

${{\Delta \; d} = {\frac{{I_{G}( {d_{2},t_{2}} )} - {I_{B}( {d_{2},t_{2}} )}}{{I_{G}( {d_{1},t_{1}} )} - {I_{B}( {d_{1},t_{1}} )}}d_{o}}},{{\Delta \; d} = {\frac{{I_{R}( {d_{2},t_{2}} )} - {I_{B}( {d_{2},t_{2}} )}}{{I_{R}( {d_{1},t_{1}} )} - {I_{B}( {d_{1},t_{1}} )}}d_{o}}}$and${\Delta \; d} = {\frac{{I_{R}( {d_{2},t_{2}} )} - {I_{G}( {d_{2},t_{2}} )}}{{I_{R}( {d_{1},t_{1}} )} - {I_{G}( {d_{1},t_{1}} )}}{d_{o}.}}$

Differential signal processing is useful to mitigate laser lightfluctuation and some specious events, such as steam absorption inducederror. Fault diagnostics in such embodiments may include voting methods.For example, if two results match, the third unmatching result is likelyan error.

FIG. 9 is a schematic drawing of a steam turbine component 400comprising a stator 410, a rotor 420 and optical fiber probes 430 and440. The stator 410 includes a fixed blade (or nozzle) 411, a fixedblade shroud 412 and/or labyrinth seals. The rotor 420, in oneembodiment, includes a shaft 421, rotating blade (or bucket) 422, ashroud, labyrinth tip seals 423, and labyrinth seals 413. Two of thethree probes 430 are located at left and right sides of FIG. 9 and arearranged radially to detect radial clearances between the fixed bladeshrouds 412 of the stator 410 and the labyrinth seals 413 on the shaft421 of the rotor 420. The probe 430 in the middle is arranged radiallyto detect radial clearances between the inner wall of the stator 410 andthe blade shroud and/or tip seals 423 of the rotor 420. The probes 440are arranged axially to detect axial clearances between trailing edgesof the stator 410 and blade shroud leading edges of the rotor 420.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. A method comprising: transmitting light through at least onetransmission optical fiber towards a target; receiving light reflectedfrom the target through at least one signal optical fiber; filtering thereceived light at least two different wavelengths; and using thefiltered light to detect a clearance variation.
 2. The method of claim1, wherein the at least one signal optical fiber comprises at leastthree signal optical fibers and wherein filtering the received light atat least two different wavelengths comprises filtering at least threedifferent wavelengths.
 3. The method of claim 2, wherein using thefiltered light to detect a clearance variation comprises determining avariation estimate at each of the at least three different wavelengths.4. The method of claim 3, wherein using the filtered light to detect aclearance variation comprises comparing the variation estimates.
 5. Themethod of claim 1 wherein transmitting light through the at least oneoptical fiber towards a target comprises transmitting light at least twodifferent wavelengths.
 6. The method of claim 1, wherein using thefiltered light to detect a clearance variation comprises converting thefiltered light to a signal using at least one photodetector.
 7. Themethod of claim 6, further comprising using the signal for steam turbineoperation control.
 8. A method comprising: transmitting light through atleast two optical fiber probes towards a target with a first one of theoptical fiber probes situated at a different distance from the targetthan a second one of the optical fiber probes; receiving light reflectedfrom the target through the at least two optical fiber probes; anddetermining a clearance variation using a differentiation between thelight received from the at least two optical fiber probes.
 9. The methodof claim 8, wherein transmitting light through each optical fiber probecomprises transmitting light at least three different transmissionwavelengths.
 10. The method of claim 9, further comprising filtering thereceived light at least three different filter wavelengths prior todetermining the clearance variation.
 11. The method of claim 9, whereindetermining the clearance variation comprises determining a variationestimate at each of the at least three different filter wavelengths ofeach optical fiber probe.
 12. The method of claim 11, wherein detectingthe clearance variation comprises comparing the variation estimates. 13.The method of claim 8, further comprising situating the first one of theoptical fiber probes at a front working point and the second one of theoptical fiber probes at a rear working point while transmitting thelight towards the target.
 14. An optical fiber probe, comprising: aplurality of optical fibers and a moisture-resistant enclosure enclosingthe optical fibers; a hydrophobic layer situated over an end of theoptical fiber probe for preventing moisture from reaching the opticalfibers; and a broadband transmission layer between ends of the opticalfibers and the hydrophobic layer.
 15. The probe of claim 14, wherein thehydrophobic layer comprises a plate comprising sapphire.
 16. The probeof claim 14, wherein the hydrophobic layer comprises a coatingcomprising aluminum oxide.